Unlike microphones used in commonplace sound recording, precision measurement microphones have an attached pre-amplifier. Direct attachment of the pre-amplifier to the microphone avoids the effects of cable capacitance and noise. However, when combined with the pre-amplifier, precision microphones require a plurality of voltages. Typically, two voltages are required, a +200 volts DC polarization voltage at almost no current and a +28 volts DC pre-amplifier power voltage at 1 mA. Because of the multiple voltage requirement, precision microphones typically require their own power supply capable delivering the required voltages.
Measurements with a precision microphone, therefore, often require three pieces of equipment: (1) a precision microphone with a built-in pre-amplifier; (2) a power supply for the microphone; and (3) an analyzer. The two voltages required by the microphone for operation are provided by the power supply. The precision microphone is also connected to the analyzer via a coaxial cable. The analyzer performs measurements on an AC audio signal produced by the microphone.
For portable precision microphone measurements, the power supply and the analyzer are usually each supplied with a battery as a power source. This is because, in general, analyzers are not constructed to allow connection of the microphone power supply directly to the battery which powers the analyzer. However, separate battery power sources for the analyzer and microphone power supply are inconvenient because of the added bulk and weight of having a separate battery for each. Also, the analyzer and microphone power supply must each be switched off to avoid running down their battery supplies. Unfortunately, it is easy to overlook switching off both the analyzer and the microphone power supply, resulting in a lack of power when needed.
Many analyzers provide a constant current source which is intended to power ICP accelerometers. These current sources permit operation of the analyzer with an ICP accelerometer as a two-wire (coaxial) system. A first wire connecting the analyzer and accelerometer is a ground wire. A second wire is a signal and power wire which conducts a constant current of 4 mA from the constant current source in the analyzer to the ICP accelerometer. The signal/power wire also conducts an AC signal produced by the accelerometer to the analyzer for analysis. ICP accelerometers do not require multiple power voltages. Therefore, a power supply is not required for ICP accelerometer applications.
The present invention provides a power supply for precision microphones and other transducers that does not require a separate battery source. The power supply according to the present invention makes use of the constant current source described above to produce the power supply voltages required by a precision microphone. Thus, the microphone power supply does not require its own separate battery power source. The analyzer's battery or AC power source provides power to both the analyzer and to the microphone power supply, the latter being supplied through the analyzer's ICP constant current source.
One advantage to the present invention is that only a two-wire connection is required between the microphone power supply and the analyzer containing the battery power source for transmission of both power and AC measurement signals. A coaxial cable can therefore be used to connect the microphone power supply and analyzer. One wire is used to conduct a constant current from the analyzer ICP current source to the power supply and to conduct an AC signal from the microphone to the analyzer. The second wire is used as a ground wire.
An additional feature of the microphone power supply is to buffer the AC measurement signal produced by the microphone. A buffer amplifier is therefore provided in the microphone power supply. The buffer amplifier is also powered by voltages produced from the constant current received from the ICP current source.
Additional features and advantages of the invention will be made apparent from the following description of the preferred embodiment, which proceeds with reference to the accompanying drawings.